Vi-style : command mode + ctrl+w reclaim (window management phase 1)

[gammons]

Summary

• Wires up the long-stubbed ModeCommand: : in normal mode opens a status-bar prompt with vim semantics (Esc/backspace-past-: cancel, Enter executes, unknown commands toast)
• Adds a : command registry (internal/ui/command.go) as the extension point for later commands; v1 ships :ws (workspace finder)
• Reclaims ctrl+w from the workspace finder so it can become the vim window-command prefix in phase 2; the help overlay now advertises :ws (workspaces also remain reachable via 1-9)
• Hardens against prompt-state leaks: SetMode centrally clears the prompt if command mode is preempted (e.g. ctrl+c quit-confirm mid-prompt)
• Docs updated (STATUS.md + wiki keybinding pages)

Phase 1 of the vim-style window management design (docs/superpowers/specs/2026-06-11-window-management-design.md); plan in docs/superpowers/plans/2026-06-11-window-management-phase1-command-mode.md.

Test Plan

• 16 new tests (statusbar prompt slot, registry dispatch, command-mode handler incl. external-preemption regression, normal-mode wiring, help-overlay entries)
• Full suite green (go test ./... -count=1), go vet clean, build clean
• Manual smoke: : shows prompt + COMMAND pill; :ws opens workspace finder; :bogus toasts; Esc/backspace cancel; ctrl+w is a no-op; ? help lists :ws and no ctrl+w

[gammons]

Superseded by #87, which combines all three window-management phases into one mergeable PR.